The prolonged mean survival time of germfree mice, compared to conventional mice, after exposure to 1000-10,000 rad whole-body irradiation has been postulated to be a function of an increased turnover time of the intestinal mucosal cells caused by the absence of free bile acids. To test this hypothesis, the diet of germ-free CFW mice was supplemented with 0.15% cholic acid for 2 weeks. The turnover of thymidine-labeled intestinal mucosal cells and the radiosensitivity to supralethal whole-body irradiation were significantly increased compared to germfree controls. There was a positive correlation between increased survival time after supralethal whole-body irradiation and slower intestinal mucosal turnover time. Germfree mice supplemented with cholic acid had intestinal mucosal turnover times comparable to those of conventionalized controls. Although cholic acid reduces the mean survival time of germfree mice after supralethal whole-body irradiation, the mean survival value is significantly greater than the conventionalized controls. Supplementing the diet of conventionalized CFW mice with cholic acid did not significantly decrease the intestinal mucosal turnover time nor did it significantly alter their radiosensitivity to supralethal whole-body irradiation. The data suggest that cholic acid is one of the microecological factors responsible for controlling the mucosal renewal rate and the mean survival time after whole-body irradiation.

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